Hydrogel core vesicles, also known as lipobeads, in which the surface of crosslinked hydrogel particles is surrounded by a lipid bilayer, are a new type of vesicles exhibiting the characteristics of both liposomes and hydrogel particles and are similar to the cytoskeleton in structure or shape. Owing to such structural characteristics, the hydrogel core vesicles are used for analysis of lipid bilayer characteristics and study of cellular models, e.g. biosensors. It can carry hydrophilic/hydrophobic materials inside/outside thereof and, when stimulation-sensitive hydrogels are used as the core, it can serve as stimulation-sensitive drug carriers capable of adjusting the release of materials included in response to an external stimulus.
Until now, two methods are known for the preparation of the hydrogel core vesicle. They are: mixing hydrogel particles with a liposome solution to induce adsorption of the liposome on the surface of the hydrogel particles; and preparing a liposome containing hydrogel-forming materials, i.e. monomer, initiator and crosslinking agent, in the inner aqueous phase and performing polymerization inside the liposome. The first method requires pretreatment of covalently bonding a fatty acid, lecithin, etc. on the hydrogel surface for effective adsorption of the lipid bilayer. In addition, the second method requires dilution with excess water to prevent polymerization of the monomers not included inside the liposome. As such, both methods involve complicated processes and are inapplicable to mass production.
Nowadays, due to artificial temperature control through air conditioning owing to change in environments or lifestyles, stresses resulting from everyday lives and skin stresses resulting from environmental pollution, frequent makeup and face washing, and natural skin aging, or the like, many people suffer dry, rough, crumbly and dark skin, etc because of loss of moisture from the stratum corneum and need skin moisturizers increasingly.
The skin is composed of largely three layers of the epidermis, the dermis and the subcutaneous tissue in order from outside. It protects internal organs of the body from change in temperature and humidity, UV light and other physical and chemical stimuli from the external environment. Especially, the epidermis plays an important role of preventing loss of water from the body. The epidermis consists of, in order from outside, the stratum corneum, the stratum granulosum, the stratum spinosum and the stratum basale. The cells of the stratum corneum function like bricks and lipid between the keratinocytes function like mortar, thus forming the skin barrier. In the keratinocytes of a healthy person, natural moisturizing factors (NMFs) which help to retain water are present at high concentrations. For example, since amino acids are water-soluble, they effectively bind to water and prevent the loss of water from the skin.
Recently, it was found out that the water holding capacity of preventing the skin from drying can be retained when the keratinocytes in the stratum basale are normally differentiated into the keratinocytes of the outermost stratum corneum. That is to say, during keratinization, the cells produce a natural moisturizing factor and intercellular lipid and, as a result thereof, the stratum corneum becomes rigid and flexible and can function as a barrier. As a person becomes older, the skin becomes drier. Physiologically, it means increased turnover time of the stratum corneum, decreased lipid synthesis ability of epidermal cells and decreased level of a moisturizing factor and lipid in the stratum corneum. Therefore, by facilitating the differentiation of keratinocytes and thereby inducing enhancement of the skin barrier, it will be possible to enhance water holding capacity of the skin and provide protection from external environment.
Until recently, humectants that absorb water or occlusive moisturizers that prevent loss of water have been used to retain water in the stratum corneum. As for the humectant, glycerin, propylene glycol, 1,3-butylene glycol, polyethylene glycol, sorbitol, and sodium 2-pyrrolidone-5-carboxylate, etc. are available, but they feel quite sticky when applied on the skin. As for the occlusive moisturizer, lipid such as ceramide, essential fatty acids, and lipid complexes, or the like have been used. But, it is difficult to keep the stability of emulsions formulation or prepare transparent gel products. Few studies have been reported on effective moisturizers featuring the functions of both the humectants and the occlusive moisturizers.